Hybrid Automatic Repeat Request (HARQ) is used in cellular communication networks such as LTE, due to the improvement in efficiency and robustness that it provides. An uplink HARQ process starts with the network node (such as an eNodeB) transmitting an uplink grant to the wireless device (such as User Equipment, UE). A certain time after receiving the uplink grant, the wireless device transmits uplink data on a shared channel according to the grant. The network node receives the transmission and attempts to decode the data. Depending on the success, the network node indicates success using an ACK (Acknowledgement) or failure using a NACK (Negative ACK) to the wireless device, implying whether a retransmission is needed or not. When a NACK is received by the wireless device, it performs a retransmission to allow the network node to attempt decoding again. Depending on the subsequent decoding result, more retransmissions can take place, up to a configurable maximum number of transmissions.
However, if the wireless device misinterprets an ACK as a NACK, it will retransmit without the network node expecting it, whereby the resources for the retransmission could have been assigned to another wireless device. This can cause serious problems for both wireless devices in terms of failed uplink transmissions which may not be resolved until the maximum number of transmission is reached.